Saturday, 8 January 2011

The hummingbird

The sight of hummingbirds hovering to refuel on the wing never ceases to astonish. Their mechanics of flight seem to defy the laws of physics. They are true masters of stationary flight and can even fly backwards — the only birds able to do so. They flap their wings around 25 times a second, though the smallest of the family (the bee hummingbird) beats at up to 90 times a second. The noise their high-frequency beating makes gives rise to their name.
Unlike other birds, hummingbirds get a portion of their lift during the upstroke of their wings. Because flying objects cannot generate lift without creating drag, birds close their wings partially and set their “angle of attack” (the wing’s incidence to the direction of flight needed for generating lift) to zero during the wasteful but necessary upstroke. That minimises the drag and conserves energy.

They then get their lift and forward thrust using a high angle of attack during the downward working stroke. By spreading their tail feathers and curling the tips of their wings back as they bring them down, the large energy-loaded vortices spilling off the leading edge of each wing can be channelled in the required downward and rearward direction to provide both lift and forward motion.
By contrast, to get the extra lift needed for hovering, hummingbirds do not simply flap their wings up and down, but oscillate them through a figure of eight pattern. By angling their bodies near to the vertical, the lift-generating vortices are thrust straight down beneath them. The hummingbird is literally buoyed on a vertical jet of air, with its head held stationary as it uses its long bill to feed.

The wings create the main vortex with a high angle of attack on the downstroke. They then flip their wings around on the upstroke, so as to create another vortex on the other side of the wing.

Pretty cool, huh?

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